Blue-hue Scattering Turbid Medium Effect

441Blue-hued scattering in Flemish Ba- roque and Dutch Golden Age paintings Sophie de Behault University of Amsterdam, Netherlands Blue-hued scattering in Flemish Baroque and Dutch Golden Age paintings Sophie de Behault University of Amsterdam ABSTRACT: 17 th century artists obtained blue colours due to wavelength-dependent scattering by using highly light- scattering pigments having specific particle size and shape. A blue-hued scattering may then appear by overlapping layers, but also by pigment mixtures (case never identified before). It may be strongly discoloured by aging degradations. It has been extensively used by Rubens and the Dutch Golden Age painters. Rubens is likely to have scientifically investigated this phenomenon. 1. INTRODUCTION: A paint layer may appear blue without containing any blue pigments. This may be explained by different phenomena. One of them is the blue-hued scattering [1]. The hypothesis put forward is that this one was obtained by artists of several periods by reducing and calibrating the particle size of pigments. This technique would have been largely exploited by 16 th century Venetian artists and transmitted to the Dutch Golden Age painters. True investigation about this topic has never been undertaken in the past. This paper aims to give the first overview of the necessary characteristics of the paint layer. 2. CREATION OF THE PHENOMENON IN PAINT LAYER: The physical phenomenon of blue-hued scattering is very distinct from other optical effects used by artists. In both the art historical context and in conservation science, the colour of a pigment is usually thought to only depend on its light absorption abilities. However, in cases of blue- hued scattering, it is obtained by selective scattering of light. In simplest cases, it appears in paint layers containing white and black pigments. White pigment particles need to have a smaller size than the wavelengths of light. As a consequence, they backscatter more intensively light with shorter wavelengths, i.e. blue light. This is wavelength- dependent scattering. It may be Rayleigh’s or Mie’s ones. However, main part of the particles found in paint layers has size causing Mie’s one. In the context of a paint layer, wavelength-dependent scattering leads blue light to penetrate less deeply than red. If a paint layer exclusively contains such white pigments, all of the wavelengths are reflected back. The colour appears white. In the presence of dark pigment, it appears blue. In this case, penetrating deeper red light encounters more dark particles. It therefore has a greater possibility of getting absorbed. As a consequence, the light reflected from paint layer appears blue. Two layer models are related to blue-hued scattering. Dark particles may be mixed together with those causing wavelength-dependent scattering (model 1) or be part of an underlying layer (model 2). Hybrid cases appear to be common. The apparition of blue-hued 1 Model 2. Overlapping layers Model 1. Pigment mixture. Fig.1 Models of paint layers causing blue-hued scattering. 442 Fig.2 Schematic reflectance curve of blue-hued scattering. scattering requires very specific conditions. These are related to characteristics of pigment particles causing wavelength-dependent scattering (size, shape, colour, polycrystallinity level) and to those of paint layers (layer thicknesses, building up, transparency, pigment concentrations and dispersions). This large amount of variables leads to many possible cases. The spectrum of blue-hued scattering has the particularity of having a very regular slope. The intensity of the reflectance decreases gradually when going from shorter to longer wavelengths. The related painting effects lead to more vivid colours than those obtained mixing the pigments available in this time period. Blue-hued scattering seems to have been used for creating green, violet and purple colours; the latter two of which were not available as contemporary pigments either as natural minerals or colorants. Example may be found in the painting Samson and Deliah by Peter Paul Rubens [2]. The blue hues of the blue-grey coat of the man cutting Samson’s hair, and the purple drapery in the background seem to have essentially been obtained by these means. The 17 th century artists also exploited blue-hued scattering to reproduce natural colour effects due to wavelength- dependent scattering, for instance, the blueness of the sky or that of the veins under one’s skin. They have similar reflectance curves. Apparently, these artists were sensitive to this similarity in hue as they used blue-hued scattering to represent the relative areas in paintings. As an example, the second model of blue-hued scattering can be easily found in the cool half-tones of flesh areas of many artists of this time. The intensity of blue-hued scattering depends on many factors and the situation in paint layer is extremely complex. However, the final reflectance can be predicted with relative accuracy by recurring to mathematical considerations. Such methods will be used in the coming researches in order to estimate blueness level due to wavelength- dependent scattering. 3. DUTCH GOLDEN AGE ART TECHNOLOGIES: The creation of colour effects due to blue-hued scattering requires high technical skills, since many factors affect the blueness of such paint layers. Lead white with the required particle size could have been obtained by synthesising tiny particles during the stack process, which was the common production method during the Dutch Golden Age. Tiny particles could have been obtained by hand-grinding pigments for a long time. Smaller particles seem to have been selected by means of gravity sedimentation, by putting the pigment in a liquid and selecting the lighter particles that settled down slower. This method could have been executed during subsequent washing of lead white pigment. This stage is necessary for high-grade lead white. The particle size causing blue light can be identified by observing light passing through the decanting water. The light is decomposed in a similar way than in paint layer. Light source appears red when observed through the liquid and blue light can be observed on top of the jar. Fig.3. Particle size distribution analyses of paint layers [3] 2 Fig.3 Pigment causing blue-hued scattering suspended in water. Light passing through appears red and blue. 443 Lead white - Particle size distribution 0 5 10 15 20 25 30 35 1 0 0 3 0 0 5 0 0 7 0 0 9 0 0 1 1 0 0 1 3 0 0 1 5 0 0 1 7 0 0 1 9 0 0 2 1 0 0 2 3 0 0 2 5 0 0 2 7 0 0 2 9 0 0 3 1 0 0 Particle Size (nm) N u m b e r P a r t i c l e s provide us with very useful information. For instance, they allow identifying cases of blue-hued scattering, but also cases obtained by means of gravity sedimentation methods. Lead white particles causing blue-hued scattering are those with diameter roughly ranging under 500 nm. High proportion of these particles leads to discriminable blue-hued scattering. The shape of the peak may also give indications about techniques used for obtaining suitable size distribution. Normally, lead white has a bimodal size distribution with a peak around 500 nm and some much larger particles, consisting of clustered monocrystals. A flat shape with larger amount of smaller particles would attest the recourse to hand-grinding. Asymmetric standard deviations from the peak lead to assume that gravity sedimentation method have been used. Another illustration of the high sophistication level reached by artists consists of their recourse to material preventing the crowding phenomenon from happening. In a situation of crowding phenomenon, light scattered on each particle interferes with light from the neighbouring particles. This leads to a decrease in intensity of the light scattering. They used extenders (calcite) and glassy materials (lakes, smalt) to this effect. Because of their very low relative refractive indices [4], they result transparent in oily binding media. They optimise then distances between light scattering particles, leading to an increase in intensity of blue-hued scattering. 4. TRANSMISSION OF THE TECHNICAL KNOWLEDGE: Examples of blue-hued scattering of both layer models have already been identified. Cases appear to be common in paintings of the Dutch Golden Age. However, no reference to these techniques has yet been found in technical literature of this period. Most probably, these techniques were professional secrets transmitted from one artist to another. Rubens, who intensively exploited blue-hued scattering in his paintings, is assumed to have investigated the physical phenomenon, and to have transmitted his technical knowledge to his school. He was recognised by his contemporaries for his scientific knowledge. He would have occupied himself with theoretical considerations about several topics related to the depiction of the reality in paintings. Considerations about blue-hued scattering are very likely to have been found in his treatises. However, these books have never been published and appear to be almost completely lost. A second type of source consists of the Rubens’ paintings. In 1608, just after his ‘grand tour’ in Italy and Spain, the young Rubens assisted Franciscus Aguilonius, a Jesuit physicist, in the writing of a treatise about optics, the Opticorum Libri Sex, in Brussels. Rubens designed the illustrations. Many clues seem to suggest that he was also involved in the theoretical contents. The Juno and Argos painting [5] was executed during the period of closest contact between 3 Fig.4 Example of particle size distribution leading to blue-hued scattering from a 17 th C. painting. The asymmetric standard deviations suggests the recourse to gravity sedimentation. 444 Fig.5 Quiringh Brekelenkam, Woman cupping, c. 1660. Detail of blueish apron. The colour change is considered to be due to saponification of lead white. The bluer part only suffered slight lead saponification and corresponds more to the original appearance of the paint. - Image courtesy of the Conservation Department, Royal Picture Gallery Mauritshuis. © R o y a l P i c t u r e G a l l e r y M a u r i t s h u i s Rubens and Aguilonius. It contains several references to his treatise and evidently forms an illustration of the theory behind blue-hued scattering. This painting is currently the object of further investigations by the author. 5. DISCOLOURATION DUE TO AGING: Blue-hued scattering is suspected to be strongly affected by aging processes. The yellowing of binding media could cause extinction of blue-hued scattering. This degradation process is caused by fluorescence, but also by increased light-absorption with the shortening of wavelengths of light. The latest would cause an off-setting of visible spectrum observed in cases of blue-hued scattering. On the other hand, an increase in the refractive index of the binding medium would have a strong effect when blue- hued scattering is obtained by overlapping paint layers (model 2). The accurate balance between scattered and absorbed light depending on wavelengths would be broken. Reflected light would tend more to the colour of the underlying layer than to a blue hue. Eventually, degradation due to lead saponification has dramatic effects. Lead saponification causes progressive consumption of pigment particles. The size reduction is faster by smaller particles, which disappear first. The consequent disappearance of particles causing wavelength-dependent scattering leads to the extinction of blue-hued scattering. Identification of discolouration cases in painting is crucial to our understanding of 17 th century paintings. Estimation of the discolouration levels will be the main goal of the future researches related to blue-hued scattering. 6. CONCLUSION: Painters of the Flemish Baroque period and Dutch Golden Age used wavelength-dependent scattering to create blue hues. The related painting effects lead to the obtainment of vivid colours or colours reproducing similar natural phenomena. Confronting physical theory with analysis of Dutch Golden Age paintings rendered possible a preliminary evaluation of the technical knowledge reached by the painters. It already clearly appears that this is higher than what is commonly thought. It is arguable that the techniques were part of the secrets transmitted from one artist to another. This could be the reason why their knowledge has progressively been forgotten with time. Rubens is suspected to have paid particular attention to the understanding of the physical phenomenon and to have used the painting Juno and Argos as an illustration of it. Several aging phenomena are suspected to lead to its extinction. Since blue-hued scattering has never been thoroughly investigated before, its survey is still at a very early stage. Further researches are already planned and will require an interdisciplinary approach. REFERENCES: [1] The phenomenon is usually referred to as ‘optical blue’, ‘opalescence’, ‘turbid medium effect’ or ‘undertone’ of the pigment. [2] Painted in 1609-1610, National Gallery of London. [3] Information obtained from SEM-bse images, by using particle size distribution analysis software. Particles were considered as spherical by simplification. [4] The refractive index of the pigment divided by that of the binding medium. [5] Painted after 1608 and before 1611, Wallraf-Richartz Museum (Cologne). 4 445 BIBLIOGRAPHY: Aguilonius, F., Opticorum Libri Sex philosophis iuxta ac mathematicis utiles, Officina Plantiniana, 1613. Boon, J.J., Weerd, J. van der, Keune, K., Noble, P. and Wadum, J. (2002). ‘Mechanical and chemical changes in Old Master paintings: dissolution, metal soap formation and remineralisation process in lead pigmented ground/intermediate paint layers of the 17 th century paintings’ ICOM-CC 13 th Triennial Meeting Preprints: 401-406, James and James. Debnath, N.C. and Kotkar, D.D. (1998). ‘Theoretical studies of light scattering power’ European Coatings Journal (n.4): 264-269 Herdan, G. (1960). Small particle statistics, Butterworths. Hulst, H.C. van de (1981). Light scattering by small particles, Dover publications. Johnston-Feller, R. (2001). Color Science in the Examination of Museum Objects, The Getty Conservation Institute. ACKNOWLEDGEMENTS: This article has been rendered possible through the courtesy and kind support of Dr. Patrick Johnson, from the Photon Scattering Group at the Foundation for Fundamental Research on Matter – FOM (Amsterdam), Dr. Klaas Jan van den Berg, Dr. Ineke Joosten, Dr. Margriet van Eikema Hommes and Dr. Luc Megens from the Netherlands Institute for Cultural Heritage (ICN), Maartje Witlox from the University of Amsterdam (UvA), Jessica Roeders from the Frans Hals Museum (Haarlem), Dr. Petria Noble and Dr. Annelies van Loon from the Royal Picture Gallery Mauritshuis (The Hague) and Dr. Ashok Roy from the National Gallery of London. 5